The system with which the present invention is concerned, comprises at least one initial device and at least one second device that must be brought into a correct configuration with the initial device in order to obtain access. Access is obtained by verifying that a coded key associated with the second device is recognized as valid by the first device, after comparison with a key found in the first device.
By way of example, in the case of payment systems with which it is possible to withdraw funds recorded on a credit card, which are referred to as an "electronic purse" and which contains the second device described above, the first device has payment terminals, each of which has one or more so-called "global" coded keys that are common to various payment terminals. Each electronic purse, in addition to any security codes, contains a memory for the monetary funds which may be withdrawn a little at a time or all in one sum, and then reloaded, e.g. by means of a bank computer terminal, as well as a memory for one or more coded keys. The description of a coded key below is given by way of clarification. One or more other keys may be used, or some other appropriate procedure may be employed.
When a payment is to be made, the electronic purse or card must communicate with a payment terminal. The payment terminal, before permitting the withdrawal of funds from the electronic purse, and possibly also for the purpose of double-checking the security codes, calculates the supposed key of the card on the basis of the global key in the terminal and on the basis of a code that identifies the individual card (for instance its serial number). The terminal then checks that this supposed key corresponds to that stored in the electronic purse card and check that it has been loaded at the time of production or issue. This calculation is carried out on the basis of a chosen algorithm that cannot be reversed. That is, if one knows the card's key and its code, it is not be possible to discover the global key.
Although this irreversible calculation does not make it possible for a third party to decipher the key and the card code in order to identify the global key, that third party could attempt to obtain the global key found in various terminals by some fraudulent means, and on the basis of that key to calculate the keys of all the cards in the system, thus obtaining or inventing their identifying codes, and then to issue false electronic purse cards which would be indistinguishable from genuine cards. Such actions would be prejudicial to the company and very costly to monitor and combat. Under extreme circumstances, these fraudulent actions could result in the need to stop using the aforesaid system.